Ionization chambers have existed for several decades. Recently, ionization chambers have been developed for various applications, such as non-destructive testing, nuclear treaty verification, geological exploration, and the like. Among these chambers, high pressure xenon (HPXe) cylindrical ionization chambers are commonly used because of the robustness and cost effectiveness of the configuration.
Conventional cylindrical ionization chambers can employ a pressurized cylindrical vessel, an inner surface of which can function as a cathode. One or more anode wires or tubes can be disposed within the chamber traversing the chamber from one end to the other along its central axis. The chamber is generally filled with compressed xenon gas, which generates electron-ion pairs in response to incident high-energy photons, such as gamma rays. An electrical field generated between the cathode and anode wire(s) or tube(s) causes free electrons, resulting from absorption of high-energy photons in xenon, to drift towards the anode wire(s) or tube(s), where the electrons are collected by the wire(s) or tube(s). The charge induced at the anode by the electrons is converted into an electrical signal, a pulse-height of which can be measured.
In ionization detectors operating as electron-only carrier devices, the pulse-height is directly proportional to a total number of electrons collected at the anode minus the charge induced by the immobile ions, which is equivalent to integration of the current induced by the electrons while they drift toward the anode. The charge induced by the immobile ions depends on the ions location inside the chamber. As a result, the height of the output signal can be dependent on a point of interaction of the high-energy photons. To minimize the effect of the uncollected positive ions, a Frisch-grid is placed inside the chamber to electrostatically shield the anode from the ions, although other techniques can be used to achieve the same effect.
Because of large sizes of the electrodes resulting in large electrical capacitance, conventional cylindrical ionization chamber detectors are sensitive to noise, electrical and acoustic, which degrades the performance of these detectors. For example, sound waves can spread within the ionization chamber medium (e.g., compressed xenon) changing the local density and dielectric constant of the xenon, which in turn can affect the energy resolution achievable by the detectors. Fluctuations of the dielectric constant induce noise signals referred to herein as acoustic noise.